The present invention relates to amplification of optical signals and in particular those signals transmitted through optical fibres and amplified utilising a rare-earth doped fibre amplifier.
Recently, the utilisation of optical fibres for communications has become increasingly popular due to their high bandwidth capabilities. The wavelengths normally utilised for optical fibre transmission have been traditionally related to the low attenuation areas of the transmission spectrum of a single mode optical fibre. Turning initially to FIG. 1, there is illustrated the spectrum of a typical attenuation rate for single mode optical fibres. The figure indicates two particular windows of interest for low loss transmission, the first being at approximately 1550 nm and the second at 1310 nm. The window at 1550 nm has become particularly popular for its low attenuation rate.
Recently the all optical rare-earth doped fibre amplifiers have also become increasingly popular for providing for the all optical amplification of an input signal. One particular form of amplifier in popular use is the Erbium doped fibre amplifier (EDFA) which has particularly strong amplification also in the 1550 nm region. FIG. 2 illustrates an example of the gain provided by a standard EDFA for different pumping powers (normalised to one with the gain also normalised to one). As can be seen from FIG. 2, the gain profile of an EDFA is highly irregular. In the past, when only a single wavelength is transmitted by an optical fibre, this is not a problem. However, recently wavelength division multiplexed (WDM) systems have been proposed and constructed with, as the name suggests, the optical fibre carrying many different channels at different frequencies or wavelengths. Unfortunately, the amplification profile of an EDFA results in each channel experiencing a substantially different gain and hence a WDM system is likely to be problematic for amplification by a EDFA amplifier unless the gain profile can be held to be substantially constant. It will, of course, be noted from FIG. 2 that an EDFA normally provides a degree of useable gain across a broad spectrum of suitable wavelengths however, as can be clearly seen from FIG. 2, the gain spectrum is xe2x80x9cswampedxe2x80x9d by the central peak.
In accordance with a first aspect of the present invention there is provided an optical amplifier for a broadband signal comprising an amplification waveguide having a gain per length of transmission that depends on the wavelength of a signal to be amplified; a first plurality of first coupling means positioned along the length of the amplification waveguide, each coupling means arranged to couple light of an associated selected wavelength from the amplification waveguide to an output means of the optical amplifier; and wherein the respective coupling means are positioned so that the gain experienced in the amplification waveguide is substantially equal for the different associated wavelengths.
The first plurality of first coupling means may be formed within the amplification waveguide.
The first plurality of first coupling means may alternatively be formed within the output means.
The output means may comprise an output waveguide.
The first coupling means may each comprise an optical grating. The grating may comprise a long period plating.
The amplification waveguide may comprise the core of an optical fibre.
In one embodiment, the output means is formed in the cladding of the optical fibre.
The output means may comprise a further core of the optical fibre.
The amplification waveguide may be formed from a rare earth-doped glass. The rare earth element may be erbium.
The optical amplifier may further comprise a second plurality of second coupling means positioned along the length of the amplification waveguide, each coupling means arranged to couple light of an associated selected wavelength from the amplification waveguide to a noise dissipation means; and the respective second coupling means are positioned after a corresponding one of the first coupling means having the same associated wavelength, so that residual signal of the same wavelength is coupled from the amplification waveguide to the noise dissipation means.
The noise dissipation means may comprise a waveguide.
The second coupling means may comprise a grating.
The second coupling means may be formed in the amplification waveguide. Alternatively, the second coupling means may be formed in the noise dissipation means.
The second coupling means may further be arranged to couple other noise from the amplification waveguide into the noise dissipation means.
In accordance with a second aspect of the present invention there is provided a method of optically amplifying a broad wavelength signal comprising the steps of amplifying the broad wavelength signal in an amplification waveguide having a gain per length of transmission that depends on the wavelength of a signal to be amplified; coupling a plurality of components of the broad wavelength signal from the amplification waveguide to an output means; and wherein each of the components is coupled from the amplification waveguide at a position such that the gain experienced in the amplification waveguide is substantially equal for the different components.
The method may further comprise the step of coupling a plurality of components of the broad wavelength signal from the amplification waveguide to a noise dissipation means; and wherein each component is coupled from the amplification waveguide to the noise dissipation means at a position after the position at which a component of the same wavelength is coupled from the amplification waveguide to the output means to couple residual signal of that wavelength from the amplification waveguide.
In accordance with a third aspect of the present invention there is provided an optical fibre having a plurality of cores, the cores having differing respective propagation constants at a predetermined wavelength, wherein a long period grating is provided in at least one of the cores, the long period grating being configured such that, in use, it matches the propagation constant of said core at the predetermined wavelength to the propagation constant of another one of the cores for coupling of light from said core into the other core.